Most mechanical components used in a variety of rolling, rotating, or sliding bearing applications, as well as those that are used in metal-cutting and—forming operations, rely strongly on high hardness and low friction surface properties of base metals for high performance and durability during use. In a dusty, sandy, and corrosive environment, high resistance to erosion and corrosion becomes important. There are numerous surface treatment methods that are currently used to enhance the near-surface properties of engineering components. Some of these methods (such as nitriding, carburizing, carbonitriding, and boriding) are theremo-chemical in nature and based on thermal diffusion of carbon, nitrogen, and boron atoms into the near surface regions of these components at high temperatures. It typically takes about 8 to 10 hours to achieve case depths of 50 to 100 micrometers in the cases of nitriding and carburizing processes; and as for boriding, the case depths are much shallower (typically 10 to 15 micrometers for the same processing time). Despite its ability to produce much harder surface layers than carburizing and nitriding, boriding is not used as extensively as the other surface treatment techniques mentioned.
There are several other surface treatment methods based on the uses of laser beams such as laser shot-peening, -glazing, -cladding, as well as ion and electron beam processes such as ion-beam deposition, electron-beam cladding, and hardening that can also be used to achieve superior surface mechanical and tribological properties. Besides these methods, there are plasma-based physical and chemical vapor deposition techniques that can also produce hard surface coatings (such as TiN, TiC, etc.) on mechanical components for improved mechanical and tribological properties. Unfortunately, all of these methods require long processing times and consume large amounts of energy.
Among the many thermal diffusion-based surface treatment processes mentioned above, nitriding and carburizing are used extensively by industry to achieve greater mechanical and tribological properties on all kinds of steel components. In the case of boriding though, progress has been rather slow and at the moment, this technique has limited uses. Just like nitriding and carburizing, boriding is a surface hardening process in which boron atoms diffuse into the near surface region of a work piece and react with the metallic constituents to form hard borides. A deep diffusion layer also exists beneath the boride layers. At present, there are several kinds of boriding methods available (such as salt-bath boriding, fluidized bed boriding, pack boriding, paste boriding, gas-phase and plasma boriding) for the production of borided surface layers. These methods are based on the uses of a variety of boron-rich solid, liquid, or gaseous media. Fluidized bed-, pack-, and paste-boriding methods use solid boron containing powders (such as B4C, amorphous boron, ferro-boron, etc.) and other compounds during the boriding process, while plasma boriding uses gaseous boron compounds in a plasma environment.
All of the boriding methods mentioned above involve a high processing temperature (typically ranging from 700 to 1000° C.). These boriding methods are most appropriate for the treatment of ferrous alloys, but nonferrous and cermet-based materials can also be treated. For example, salt-bath boriding of steel substrates can be done in a complex salt bath typically consisting of 60 to 70 wt % borax, 10 to 15 wt % boric acid, and 10-20 wt % ferro-silicon or boron at temperatures ranging from 800 to 1000° C. Boriding of a low carbon steel substrate for 5 to 7 hours in such a salt-bath may result in 7 to 10 micrometer thick borided surface layers.
During boriding of steel and other metallic and alloy surfaces, boron atoms diffuse into the material and form various types of metal borides. In the case of ferrous alloys, most prominent borides are: Fe2B and FeB. (Fe3B may also form depending on the process parameters). Some of the boron atoms may dissolve in the structure interstitially without triggering any chemical reaction that can lead to boride formation. Iron borides (i.e., Fe2B and FeB) are chemically stable and mechanically hard and hence can substantially increase the resistant of base alloys to corrosion, oxidation, adhesive, erosive, or abrasive wear. Process conditions (such as duration of boriding, ambient temperature, type of substrate material and boriding media) may affect the chemistry and thickness of the borided surface layers. Due to the much harder nature of borided layers, boriding has the potential to replace some of the other surface treatment methods like carburizing, nitriding and nitrocarburizing.
Boride layers may achieve hardness values of more than 20 GPa depending on the chemical nature of the base materials. TiB2 that forms on the surface of borided titanium substrates may achieve hardness values as high as 30 GPa; ReB2 that forms on the surface of rhenium and its alloys may achieve hardness values as high as 50 GPa, while the hardness of boride layers forming on steel or iron-based alloys may vary between 14 GPa to 20 GPa. Such high hardness values provided by the boride layers are retained up to 650° C. Since there is no discrete or sharp interface between the boride layer and base material, adhesion strengths of boride layers to base metals are excellent. With the traditional methods mentioned above, boride layer thicknesses of up to 20 micrometer can be achieved after long periods of boriding time at much elevated temperatures. In addition to their excellent resistance to abrasive, erosive, and adhesive wear, the boride layers can also resist oxidation and corrosion even at fairly elevated temperatures and in highly acidic or saline aqueous media.
Materials that are most suitable for boriding include all types of ferrous metals and alloys like low- and high-carbon steels, low- and high-alloy steels, tool steels, bearing steels, stainless steels, precipitation hardening steels, carburized, nitrided, and carbonitrided steels, and cast irons. Non-ferrous metals and their alloys like aluminum, hafnium, vanadium, nickel, chromium, cobalt, titanium, tantalum, zirconium, tungsten, niobium, molybdenum, rhenium, magnesium, and their alloys in particular nickel-based and cobalt-based superalloys, cobalt-chrome alloys, tungsten and sintered carbides and/or cermets can also be borided.
Because of their impressive mechanical, tribological, chemical and corrosion properties, borided surface layers can be used in a large variety of industrial applications. In metal-forming dies, they can be used to protect the critical surface finish or profiles of all kinds of dies (such as punching dies, drawing dies, bending dies, hot forming, and injection moulding dies, forging dies, die-casting, extrusion dies, embossing dies, deep drawing and impact extrusion dies). They can also be used in insertion pins, rods, plungers, bushings, botts, nozzles, pipe bending devices, guide rings, sleeves, mandrels, swirl elements, clamping, chucks, guide box, metal casting inserts, orifices, springs, balls, rollers, discs, valve components and fittings, plugs, chain components, etc. They will be extremely well-suited for stainless steel and other metallic-based mechanical shaft seals used in pumping all kinds of fluids, pulps, powders, slurries, in chemical and mining industries. In the automotive or transportation fields, they can prevent seizure, galling and scuffing-related failures under severe operating conditions, and eliminate oxidative and corrosive degradation of a large variety of engine components. They can also be used in a variety of gear drives (such as bevel gears, screw and wheel gears, helical gear wheels), including gears, ball and roller bearings, tappets, valves and valve guides, power train components, piston pins, rings and liners, fuel injector components for liquid (gasoline and diesel) and gaseous fuels (like hydrogen, propane, natural gas) and other types of mechanical components in all classes of moving mechanical systems that experience heavy loading, high speeds, erosive, corrosive, and oxidative media and elevated temperatures. Other potential applications include cold and hot forging tools, extrusion tools, press tools, glass industry tools, metal-casting tools, cutters, razor blades and shaving machines, impellers used in pumps, mixers used in chemical and mining industries, ball joints, turbine and helicopter blades that are subject to sand erosion, compressor parts and foil bearings, invasive and implantable medical devices such as hip and knee joints made out of titanium, zirconium, cobalt-chrome, stainless steel, and other specialty metals and their alloys. Because of the high boron content of their near surfaces, borided surfaces can also provide an excellent substrate for the deposition of diamond and diamondlike carbon films on metallic substrates. In most cases, diamond is difficult to deposit on steel substrates; but after the boriding process such surfaces could be ideal for the nucleation and growth of crystalline diamond and amorphous diamondlike carbon films. Such duplex boride-diamond or diamondlike carbon treated surfaces can be ideal in many machining, metalforming, sealing, and biomedical implant applications.
Despite their abilities to produce much harder surface layers and superior components over other methods, conventional boriding methods mentioned above are not used extensively by industry at the moment. There are substantial problems that hinder their wider use. Some of these problems include: high-cost, long processing time, toxic emissions/byproducts, mechanical and structural degradations, and poor surface condition or finish after the boriding process. For all of these reasons, it would be desirable to develop a new and improved boriding method that is fast, inexpensive, safe, and applicable to a wide range of materials.